The invention relates to a double-layer fabric for the sheet forming section of a papermaking machine made up of two layers of transverse threads which are interwoven with a single system of longitudinal threads.
Fabrics for the sheet forming section of a papermaking machine in which the transverse threads are arranged in two layers and are interwoven with a single system of longitudinal threads are generally designated as double-layer sheet forming woven fabrics or, briefly, double-layer forming fabrics. Double-layer forming fabrics brought about a substantial improvement in the sheet forming characteristics as opposed to a single-layer forming fabrics. The longitudinal stability has been substantially improved by the high degree of filling of the longitudinal threads on the order of 95-110%. The dewatering characteristics of these forming fabrics are also more favorable because there are not provided any large openings passing through in the vertical direction, and the water is conducted away from the paper pulp through numerous fine, slot-like openings which are predominantly arranged in an oblique direction.
It is possible in the case of double-layer forming fabrics to select the dimensions, the material characteristics and the shape of crimping of the two weft systems to be different. For reasons having to do with marking, fine transverse threads of relatively short floating length may be used for the paper side, while thick transverse threads having long floatings may be used for the running side. It is possible, furthermore, to utilize different materials for the running side. It is customary for the transverse threads of the running side to consist alternatingly of polyester and abrasion-resistant polyamide.
A two-layer forming fabric is understood to be, by contrast, a fabric for the sheet forming section of a papermaking machine, which consists of an upper fabric and a lower fabric, each of which are woven complete in themselves from longitudinal threads and transverse threads with upper fabric and the lower fabric being connected either by means of specific binding threads or by means of weaving of the longitudinal or the transverse threads of the upper fabric into the lower fabric or vice versa. The upper fabric is fine-meshed in nature. It consists of a plurality of thin threads and has little stability in itself. The lower fabric is coarse and consists of thick threads. It imparts to the woven fabric the required longitudinal and transverse stability. The binder threads may extent in a longitudinal direction, in a transverse direction or simultaneously in both directions.
The binder thread is the weak point of these two-layer forming fabrics. It frequently is destroyed prematurely due to abrasion from the running side or due to internal friction between the two layers of woven fabric. U.S. Pat. No. 4,592,396 offers a remedy for this. This forming fabric is provided on the running side with transverse threads arranged pairwise and the binding thread no longer lies on the running surface but is nipped by a pair of transverse threads and retained over against the longitudinal thread crossing thereabove. In the case of two-layered forming fabrics, the connection of the two layers to one another is not strong enough for avoiding mutual relative movement. In the event of alternating deflection of the forming fabric via externally-disposed and internally-disposed guide rollers, there results internal friction between the two layers of woven fabric.
Due to the high proportion of filler materials in paper for writing and in paper for printing, the abrasive particles of the filler get in between the two layers of woven fabric. The face between the two layers of woven fabric is also highly abraded on account of the relative movement and the filler material. The binding threads are often prematurely chafed through at the points of contact with the layers of woven fabric. As a result of this, the layers of woven fabric are separated and the forming fabric becomes unserviceable.
U.S. Pat. No. 4,739,803 describes a forming fabric, which is similar to the double-layer forming fabric but has the finer surface structure of a two-layer forming fabric. The structure on the paper side is made to be very fine due to a specific longitudinal thread path and due to the number of transverse threads having been doubled. The running side of this forming fabric is, by contrast, coarse and very open. Furthermore, successive transverse threads on the paper side are supported in a different manner in that one transverse thread is carried in saddle-like manner by the longitudinal thread, so that it is lying precisely in a transverse direction in the woven fabric, while the next-following transverse thread is supported by two adjacent longitudinal threads, one of said longitudinal threads rising to the forming fabric surface and the other one descending, so that the supporting of the transverse thread takes place in a shear-like manner and the transverse thread floating is turned in a manner such that it lies at an angle with respect to the floatings of the transverse threads supported in a saddle-like manner. In this way, the marking characteristics of the forming fabric are substantially improved. The number of transverse threads in the case of this forming fabric is twice as high on the paper side as on the running side. It is in that way possible to achieve substantially higher numbers of transverse threads on the paper side than in the case of a conventional, double-layer forming fabric (with a similar degree of fineness of the longitudinal threads). The maximum possible number of transverse threads is limited in the case of double-layer forming fabrics on account of weaving technique because the longitudinal threads always alternate between the upper and lower layer, so that only a limited total number of transverse threads can be shot-in. With a similar total number of transverse threads, the forming fabric according to U.S. Pat. No. 4,739,803 has more threads on the paper side than a conventional forming fabric because the number of threads on the running side is only half as large as that on the paper side.
According to EP-A-O No. 0 085 363, the marking characteristics and the retention of the fibers are improved in that an additional supporting thread is inserted without binding it in subsequent to each structural thread in a single-layer or on the paper side of a double-layer forming fabric. This serves to reduce the mesh size and the fibers of the paper pulp are better supported. For reasons having to do with weaving technology, it is not possible, however, to still weave-in further, additional threads once a maximum possible thread density of the double-layer forming fabric has been attained. The number of transverse threads of the original weave construction must be reduced by about one-third in order to arrive, with additional threads, at about the same total number of transverse threads.
It has turned out in practice that the forming fabrics according to the EP-A-O No. 0 085 363 and U.S. Pat. No. 4,739,803 substantially improve the forming fabric marking and retention. Care must be taken, however, upon weaving of these double-layer woven fabrics that the transverse threads are lying perfectly one above the other. This presents problems because the position of the upper and the lower threads relative to one another changes on account of the crimping exchange phenomena occurring during heat setting. During weaving, the transverse threads are still laid in substantially uncrimped state into the composite structure of the longitudinal threads. The longitudinal threads are looped around the two layers of the inserted transverse threads and initially form the outermost points of the woven fabric on the paper side as well as also on the running side. It is only upon heat setting at very high longitudinal tension and with the action of heat that the longitudinal threads are stretched and force the transverse threads to crimp. There is thus formed, for instance, the long transverse thread floating on the running side, which in the finished forming fabric extends out of the plane of the woven fabric and defines the abrasion surface of the forming fabric. This is then called the weft runner. At the same time the longitudinal thread is on account of the high longitudinal tension urged to the inside of the woven fabric. It is in that way protected from abrasion on the running side or is at least exposed to abrasion only after the volume of the transverse threads has already largely been spent. Simultaneously with these crimp exchange phenomena, the transverse threads slip above one another, and it is extremely difficult to weave the woven fabric in a manner such that all of the transverse threads are lying perfectly above one another over the full length and width of the forming fabric.
In the case of a forming fabric having a width of 9 meters and having 70 longitudinal threads/centimeter, the forming fabric has, in all, 63,000 longitudinal threads. When the number of transverse threads is 30 transverse threads/cm in the upper layer and 30 transverse threads/cm in the lower layer a forming fabric having a length of 45 meters has 135,000 transverse thread pairs in all. In the case of a 7-harness weave, the result is 1215 million transverse thread floatings which must all be lying one neatly above the other. If slippage of one upper transverse thread relative to a lower one occurs at only a single location, then this location will cause, at the high operating speeds of the forming fabrics, an inadmissible marking on the paper web, and the entire forming fabric will become unserviceable.
It is extraordinarily difficult to attain, with so many longitudinal threads and so many transverse thread pairs, a perfect super-position which really does remain the same. The fabric tension must already upon weaving be maintained precisely over the full width of the loom and, of course, over the full length of the forming fabric. Furthermore, all of the transverse threads must have exactly the same characteristics in the cold state upon weaving as well as also at the high temperatures during the setting procedure. For the woven fabric to become resistant to abrasion, polyester threads and polyamide threads are alternatingly woven into the fabric on the running side. These two working materials exhibit very different behavior, especially upon setting. If a good super-position of the upper transverse thread above the lower transverse polyester thread is attained, then the next upper transverse thread slips off of the transverse polyamide thread disposed below it. Minor offsets already bring about very distinct markings in the paper. Furthermore, the polyamide monofilament crimps under tension and the action of heat in a different way than does the polyester monofilament. This depends upon temperature and tension. The result is that the longitudinal thread on the running side, when the polyamide thread is woven in, is embedded less deeply within the woven fabric than when the polyester threads are interlaced. While it is possible to reduce these crimping differences by selecting physical characteristics that are as favorable as possible, it is nevertheless not possible to entirely avoid these differences in the inner structure of the forming fabric.
These problems are encountered in the production of all double-layer forming fabrics. They are particularly serious in the case of the forming fabric constructions according to EP-A-O No. 0 085 363 and U.S Pat. No. 4,739,803 wherein the inner structure is from the very outset less balanced due to the absence of every second transverse thread on the running side. Slipping off of the transverse threads is a phenomenon that is particularly marked in the case of these woven fabrics.
In addition, the number of transverse threads on the running side is less in the case of these forming fabrics and on account of that the distances between transverse threads are substantially greater than in the case of double-layer forming fabrics having a like number of transverse threads in the upper and in the lower layer. Considering that at the high speed of modern papermaking machines for printing paper of greater than 1000 m/min. hydrodynamic pressure waves, which go out from the running side, have a decisive influence on marking, the large distance between transverse threads on the running side has a disadvantageous effect in regard to marking. The intensity of the pressure waves depends upon the distance between successive transVerse threads. When woven fabrics with only half the number of transverse threads on the running side are used, it sometimes happens that insupportably extensive transverse thread markings become visible in the paper web. The density of the lines of the marking in the paper web is not such as to correspond to each lower transverse thread but, rather, only to every second transverse thread on the running side; this is the transverse polyamide thread. The causes for this are the above-described differences in the super-positioning (slipping-off) and the crimping differences between polyamide and polyester. In some instances, the otherwise so interesting advantages which these forming fabrics provide for are in this way brought to nothing.
Another negative factor encountered by woven fabrics having a low number of transverse threads on the running side is entrainment of water. These woven fabrics are closed on the paper side but very open at the bottom. Larger quantities of water are retained in the open space of the woven fabric openings and are then flung-out upon the deflection over guide rollers.
It is known from German application No. DE-A-31 46 385 to improve retention in the case of a double-layer papermaking machine forming fabric in that the longitudinal threads on the paper side and/or on the running side are conducted at least over some distance in parallel pairwise. The parallel guiding of the longitudinal threads is of a nature such that the threads of the paper web are supported by way of an areal structure, thus avoiding the distinct impressions of individual longitudinal threads upon the paper web.
EP-A-O No. 117 856 describes a two-layer papermaking machine forming fabric, in which the warp threads of the lower layers are interwoven pairwise in parallel over the entire extent thereof. It is made possible thereby to produce, on the one hand, double-layer papermaking machine forming fabrics and, on the other hand, two-layer papermaking machine forming fabrics with warp threads of like diameter in the same warp thread density.
A two-layer papermaking machine forming fabric is known from German Application No. DE-A-34 45 367, in which the lower woven fabric contains, for the purpose of avoiding high tensions in the binding threads, two groups of transverse threads, the first group of which being interwoven in a 1:1-weave and the second group in a 1:n-weave with the warp threads of the lower Woven fabric, with successive transverse threads of varying weave construction, which define double crimpings on the inner side of the lower woven fabric, contacting, and with the long n-crimps defining on the outer side a plane of wear. In that respect, n denotes an odd number greater than 1.